Electric discharge lamp



March 30, 1954 v. J. FRANCIS 2,673,944 ELECTRIC DISCHARGE LAMP Filed Dec. 19, 1949 Inven kor:

\lic lror' James Fvancis,

- His A lr irovneg.

Patented Mar. 30, 1954 ELECTRIC DISCHARGE LAMP Victor James Francis, North Wemhley, England, assignor to General Electrie Company, a corporationof N ew' York:

Application December 19, 1949, Serial N 0. 133,780

Claims priority, application Great Britain December 23, 1948 7 Claims. 1

This invention relates to electric discharge lamps of the type comprising within a sealed ens velope a gaseous or vaporizable discharge medium and at least one pair of solid electrodes between which a constricted discharge is adapted to pass when the lamp is in normal operation. The requirement of a constricted discharge implies that the pressure of the discharge medium in nor,- mal operation is greater than 100 mm. mercury.

The lamps of this type which are most widely used at present are mercury vapor discharge lamps, in which the discharge medium, usually referred to as the filling, consists wholly or mainly of mercury which in normal operation of the lamp is vaporized to attain a pressure approaching or exceeding, often considerably, one atmosphere. Lamps of the type specified having a. fill-1 ing of permanent gas at a pressure exceeding 100 mm. mercury have also been proposed, but such lamps have not yet come into general use. One of the disadvantages of such high pressure gas-filled lamps which has prevented their general use is that the discharge is very unstable, the position of the discharge column shiftingrapidly and erratically between the electrodes. Another disadvantage is that the efiiciency of the discharge is low compared with that of a mercury vapor discharge, but this maybe relatively unimportant in some cases, for example, for obtaining radiation of particular spectral distribution; there is, however, one important ex ception to be made in respect of this'second 'disad vantage, and that is in the case of xenon. A high intensity xenon discharge is capable of giv ing an efficiency which is not so low as to be impracticable for lighting purposes, and, moreover, the color ofthe light emitted is much better than that of a mercury vapor discharge for general lighting purposes.

In order to overcome the first said disadvantage and to render the use of a high intensity xenon discharge practicable for lighting purposes, it has already been proposed to include the Xenon within a narrow tubular envelope to immobilize the discharge, but the envelope then necessarily requires water cooling in operation.

The principal object of this invention is to overcome the disadvantage of instability in a different way so as to render practicable, without necessarily requiring water cooling, the use in discharge lamps of the type specified of fillings of permanent gas, and especially xenon. Still A feature of the invention. is the inclusion within the lamp envelope together with the permanent gas of a quantity of vaporizable metal such that in operation of the lamp the pressure of the metal vapor is appreciable but is, or may be controlled by cooling of the lamp to be, so much less than that of the permanent gas that the light emitted from the lamp is due. to an appreciable extent to the presence of the permanent gas.

It has been found that by the inclusion of metal vapor in the filling in this way the discharge is rendered much more stable, but the reason for this is. not yet fully understood.

Suitable vaporizable metals for. use in accordance with this invention are, for example, mercury, cadmium, zinc, tellurium, thallium, indium, gallium, or lead. It is desirable in any particular case that the ionization potential of the metal should be less than that of the permanent gas, and it appears that. a suitable pressure for the metal vapor to attain is about 1 to 10 mm. mercury.

The use of an addition ofmetal vapor in accordance with this invention may also help to reduce the secondary disadvantage aforesaid; thus the presence of metal vapor, especially mercury, in the filling may increase the efficiency i of the discharge as a whole- Moreover, the part of the light output dueto the metal vapor may serve to modify the color of the light due to the permanent gas; thus in the case of a xenon-mercury filling, the blue light from the mercury may combine with the pinkish light from the xenon to produce substantiallywhite light. The pressure attained by the metal vapor in operation may then be arranged to be. somewhat higher-than the minimum required merely to'stabilize the discharge and may further objects and advantages of the invention will appearirom the following description of species thereof.

approach 100 mm. mercury.

The invention has particularly useful application in the case of high intensity lamps of the type specified having an approximately spherical envelope, a discharge path (distance between the electrodes) much less than the diameter of the envelope, and adapted to dissipate in normal operation more than 500 watts per centimeter of the discharge path. Such lamps with mercury or mecury-cadmium fillings have been much used for optical projection and fiood lighting purposes, and for such purposes the light from a high intensity xenon discharge is very suitable.

In the drawing accompanying and forming part of this specification, a high intensity lamp of this kind is shown in an elevational schematic view.

Referring to the drawing, the lamp comprises an approximately spherical quartz envelope i having tungsten rod electrodes 2 and 3 sealed into the envelope at diametrically opposed points by means of quartz-to-metal seals of the multiple molybdenum foil type included in the stems 4 and 5 respectively, of the lamp. The stems 4 and 5 extend in opposite directions from the envelope l and are provided with metal bases 5 and 1, respectively, adapted for connection with suitable terminals of a suitable source of electrical energy for the lamp.

Preferably the lamp is adapted to dissipate 2500 watts in normal operation and has an approximately spherical quartz envelope I of about 60 mm. in diameter with walls 2 to 3 mm. thick. The electrodes are positioned so as to form an arc gap of 15 mm. at the center of the envelope I. The lamp envelope is filled with xenon to a pressure, when the lamp is cold, that is, at room temperature, of 600 mm. mercury and contains sufficient mercury, about milligrams, to establish a mercury vapor pressure of about 100 mm. mercury in normal operation, i. e. when the lamp is dissipating 2500 watts without forced cooling. The color of the light emitted by the lamp in operation may be controlled if required by reducing the mercury vapor pressure by forced air cooling of the envelope.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge lamp of the high pressure type comprising a spherical vitreous envelope having therewithin solid electrodes spaced apart a distance less than the diameter of the envelope and a filling comprising permanent gas at greater than 100 mm. pressure at room temperature together with a quantity of vaporizable metal whose ionization potential is less than that of the gas and means to supply to said lamp at least 500 watts of electrical energy per centimeter of length of the discharge path between said electrodes in operation, the pressure of said gas, the quantity of vaporizable metal in said envelope, and the heat dissipating characteristics in still air of said envelope with respect to the energy input to said lamp being such that when the lamp is dissipating more than 500 watts per centimeter of the discharge path the discharge is constricted and the pressure of the metal vapor is at least 1 to 10 mm. but is so much less than that of the permanent gas that the light emitted from the lamp is due to an appreciable extent to the presence of the permanent gas and the discharge in said lamp is stable during operation.

- 2. An electric discharge lamp according to claim 1 wherein the said permanent gas is xenon.

3. An electric discharge lamp according to claim 1 wherein the said permanent gas is xenon and the said vaporizable metal consists of one or more metals of the group consisting of mercury, cadmium, zinc, tellurium, thallium, indium, gallium, or lead.

4. An electric discharge lamp according to claim 1 wherein the permanent gas is xenon and the vaporizable metal is mercury limited to an amount to be all evaporated in operation of the lamp.

5. An electric discharge lamp according to claim 1 wherein the permanent gas is xenon at 600 mm. pressure at room temperature and the vaporizable metal is mercury and is limited to an amount such as to give a vapor pressure of about 100 mm. in said operation of the lamp.

6. A high pressure electrical discharge lamp comprising a sealed vitreous spherical envelope about mm. in diameter having therewithin solid electrodes disposed at diametrically opposite points and spaced apart about 15 mm., a filling of xenon therein at a pressure of approximately 600 mm. at room temperature and about 10 milligrams of mercury therein to produce a mercury vapor pressure not greater than about mm. when the lamp is dissipating 2500 watts and means to supply 2500 watts of electrical energy to said lamp whereby the xenon is luminosity producing and the discharge between said electrodes is stable during operation of the lamp.

7. A high pressure electrical discharge lamp comprising a sealed quartz spherical envelope about 60 mm. in diameter with walls 2 to 3 mm. thick having therewithin tungsten electrodes at diametrically opposite points and spaced apart about 15 mm. at the center of the envelope, a filling of xenon therein at a pressure of approximately 600 mm. at room temperature and about 10 milligrams of mercury therein to produce a mercury vapor pressure not greater than about 100 mm. when the lamp is dissipating 2500 watts and means to supply 2500 watts of electrical energy to said lamp whereby the xenon is luminosity producing and the discharge between said electrodes is stable during operation of the lamp.

VICTOR JAMES FRANCIS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,087,735 Pirani July 20, 1937 2,272,467 Kern Feb. 10, 1942 2,275,739 Dellian Mar. 10, 1942 2,317,061 Inman Apr. 20, 1943 2,363,531 Johnson Nov. 28, 1944 2,404,953 Francis July 30, 1946 

